1. Field of the Invention
The present invention relates to a device for reading information in a set of electronic components connected to a communication bus in order to send that information over the communication bus. It also relates to a corresponding method and to an application of that method to reading information contained in a matrix of pixels.
More precisely, the invention relates to a device for reading information in a set of electronic components connected to a communication bus in order to send that information over the communication bus, the system including means for reading values held in at least some of the electronic components holding a value to be sent and an arbiter circuit for successively sending the read values over the communication bus.
2. Description of the Prior Art
Published European patent EP 1 150 250 describes a device for reading information in a set of electronic components connected to an asynchronous communication bus and for coding that information in order to send it over the communication bus.
More specifically, the device described in the above patent reads values contained in a matrix of pixels and time codes those values. A signal that is distributed to all the pixels transfers a reference value that changes over time. This reference value is compared to a local value contained in each pixel. If the reference value and the local value are equal, the pixel sends a pulse over the asynchronous communication bus. The pulse codes the position of the pixel in the matrix and the time of sending the pulse codes the local value contained in the pixel.
The major drawback of the mode of transmission described in the above patent is that values contained in pixels holding a value to be sent are sent without arbitration, causing collisions on the asynchronous transmission bus. This leads to a loss of information. A first solution to this problem is to maintain activity on the asynchronous communication bus at a relatively low value, in practice less than 10%, in order to limit the risk and therefore the occurrence of collisions.
In the more general field of sending information extracted from electronic components connected to a communication bus, published U.S. Pat. No. 5,404,556 describes another solution. According to that patent, successive sending of the read values over the asynchronous communication bus is subject to arbitration. To be more precise, arbitration is effected by means of a binary decision tree that at any given time selects a single electronic component from all the electronic components that are holding a value to be sent and authorizes that component to send its value on the asynchronous communication bus at that time.
The above patent therefore solves the problem of collisions. However, the order in which the electronic components are authorized to send the value they are holding is indeterminate; it depends on the structure of the binary decision tree and on the propagation delays internal to that binary tree. Thus the method described cannot process sequentially all of the components that at a given time are holding a value to be sent. As a result of this, a component requesting access to the communication bus continuously, for example following a fault, could compromise the correct operation of this communication principle. On the other hand, sequential selection of components requiring access to the communication bus at a given time ensures fair sharing of that communication bus.
The invention seeks to solve the above problem by providing a device and a corresponding method that ensure systematic selection of all electronic components holding information to be sent, at the same time as maintaining a zero risk of collisions.